Steering controlling device

ABSTRACT

A steering controlling device comprising a power source, a motor connected to a steering system, a motor driving circuit converting a power source voltage to a predetermined voltage and applying the voltage to the motor, and a controlling means for controlling the motor driving circuit, wherein the controlling means controls the motor driving circuit so as to boost the power source voltage when a rotational speed of the motor is a predetermined value or more, whereby a sufficient motor output torque is obtainable under various steering conditions in use of a converter, an efficiency is improved by suppressing a frequency of operating the converter, and an appropriate fail safe treatment is conducted when the converter is disordered.

This is a continuation of application Ser. No. 10/998,579 filed Nov. 30,2004, now U.S. Pat. No. 7,294,985 which in turn is a continuation ofapplication Ser. No. 09/962,136, filed Sep. 26, 2001, now U.S. Pat. No.6,987,371. The entire disclosure of the prior applications, applicationSer. Nos. 10/998,579 and 09/962,136 are considered part of thedisclosure of the accompanying continuation application and are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a steering controlling device forautomobiles and so on.

2. Discussion of Background

In a conventional technique, an electric power steering device disclosedin Japanese Unexamined Patent Publication JP-A-7-76280 is known, whereinenhancement of an efficiency of power conversion is aimed. FIG. 5 is ablock chart illustrating a structure of a conventional device. In theconventional electric power steering device, when a vehicle speed islow, it is judged that a load of a motor is high, whereby a voltage ofpower source is boosted, and the motor is driven by the boosted voltage.

However, since the conventional device is constructed as describedabove, the power source is boosted when a vehicle speed is low, wherebya frequency of using the converter is increased. Therefore, there areserious problems that an efficiency is lowered by a loss in theconverter, a fuel consumption of a vehicle is deteriorated byover-discharge of a battery or an increment of generating power, and aneffect of an electric power steering system, which is originally aimedto improve the fuel consumption by its high efficiency, is notobtainable.

Further, in the conventional device, failures of the converter are notconsidered.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above-mentionedproblems inherent in the conventional technique and to provide asteering controlling device which generates a sufficient output torqueof a motor under various steering conditions, suppresses a frequency ofoperation of a converter, and improves an efficiency, in use of theconverter. Another object of the present invention is to enable a failsafe treatment when the converter is disordered.

According to a first aspect of the present invention, there is provideda steering controlling device comprising: a power source; a motorconnected to a steering system; a motor driving circuit converting apower source voltage to a predetermined voltage and applying theconverted voltage to the motor; and a means for controlling the motordriving circuit, wherein the controlling means controls the motordriving circuit so as to boost the power source voltage when arotational speed of the motor is a predetermined value or more.

According to a second aspect of the present invention, there is providedthe steering controlling device, wherein the controlling means operatesthe rotational speed of the motor based on an electric current and animpressed voltage of the motor.

According to a third aspect of the present invention, there is provideda steering controlling device comprising: a power source; a motorconnected to a steering system; a motor driving circuit converting apower source voltage to a predetermined voltage and applying theconverted voltage to the motor; and a means for controlling the motordriving circuit, wherein the controlling means controls the motordriving circuit by controlling an electric current of the motor so as tobe a predetermined target value and boosting the power source voltagewhen the electric current of the motor is lower than the target value soas to drive the motor.

According to a fourth aspect of the present invention, there is provideda steering controlling device comprising: a power source; a motorconnected to a steering system; a motor driving circuit converting apower source voltage to a predetermined voltage and applying theconverted voltage to the motor; and a means for controlling the motordriving circuit, wherein the controlling means controls the motordriving circuit by controlling an electric current of the motor so as tobe a predetermined target value and boosting the power source voltagewhen an impressed voltage applied to the motor exceeds a predeterminedvalue so as to drive the motor.

According to a fifth aspect of the present invention, there is providedthe steering controlling device, wherein the motor driving circuit iscontrolled to boost the power source voltage in order to drive the motorwhen a referee condition for boosting the power source voltage issatisfied for a predetermined time or more.

According to a sixth aspect of the present invention, there is providedthe steering controlling device, wherein the motor uses a magnetic fieldby a permanent magnet.

According to a seventh aspect of the present invention, there isprovided the steering controlling device, wherein the motor drivingcircuit includes at least a first converter connected to the powersource and a bridge circuit connected to the motor.

According to an eighth aspect of the present invention, there isprovided the steering controlling device, further comprising: a secondconverter boosting an output voltage from the first converter in orderto drive switching elements, forming the bridge circuit.

According to a ninth aspect of the present invention, there is providedthe steering controlling device, wherein the switching elements on aside of the power source among the switching elements, forming thebridge circuit, are driven by the second converter.

According to a tenth aspect of the present invention, there is providedthe steering controlling device, wherein the controlling means monitorsat least an input voltage to the first converter.

According to an eleventh aspect of the present invention, there isprovided the steering controlling device, wherein the controlling meansmonitors at least an output voltage from the first converter.

According to a twelfth aspect of the present invention, there isprovided the steering controlling device, wherein the controlling meanscontrols an output voltage from the first converter so as to be apredetermined target value.

According to a thirteenth aspect of the present invention, there isprovided the steering controlling device, wherein the controlling meanscontrols so that the boosted voltage is a predetermined value or less.

According to a fourteenth aspect of the present invention, there isprovided the steering controlling device, wherein the controlling meansstops a first boost operation when the first converter is judgedabnormal.

According to a fifteenth aspect of the present invention, there isprovided the steering controlling device, wherein the controlling meansstops an application of a current to the motor when the first converteris judged abnormal.

According to a sixteenth aspect of the present invention, there isprovided the steering controlling device, wherein the power source isconnected to the motor driving circuit through a switching means, andthe controlling means opens a contact point of the switching means whenthe first converter is judged abnormal.

According to a seventeenth aspect of the present invention, there isprovided the steering controlling device, wherein a duty ratio ofdriving the switching elements of the bridge circuit is 100% when thefirst converter is operated to boost the voltage.

According to an eighteenth aspect of the present invention, there isprovided the steering controlling device, wherein terminals of diodesforming the first converter are short-circuited when the first converteris not operated to boost the voltage.

According to a nineteenth aspect of the present invention, there isprovided the steering controlling device, wherein the diodes forming thefirst converter are parasitic diodes of MOSFET.

According to a twentieth aspect of the present invention, there isprovided the steering controlling device, wherein the controlling meanscontrols the electric current of the motor so as to be a predeterminedtarget value by driving at least one of the first converter and thebridge circuit by pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanied drawings, wherein:

FIG. 1 is a block chart schematically illustrating a steeringcontrolling device according to Embodiment 1 of the present invention;

FIG. 2 is a flow chart illustrating an operation of the steeringcontrolling device according to Embodiment 1 of the present invention;

FIG. 3 is a graph explaining characteristics of a motor of the steeringcontrolling device according to Embodiment 1 of the present invention;

FIG. 4 is a block chart schematically illustrating a motor drivingcircuit of the steering controlling device according to Embodiment 1 ofthe present invention; and

FIG. 5 is a block chart schematically illustrating a conventionalsteering controlling device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed explanation will be given of preferred embodiments of thepresent invention in reference to FIGS. 1 through 4 as follows, whereinthe same numerical references are used for the same or similar portionsand descriptions of these portions is omitted.

Embodiment 1

FIG. 1 illustrates an embodiment of the present invention. Numericalreference 1 designates a battery; and numerical reference 2 designates afirst converter for boosting a voltage of the battery 1, the firstconverter is formed by a choke coil 21, a first switching element 22, asecond switching element 23, and a capacitor 24. Numerical reference 3designates a motor connected to a steering system (not shown), the motoris a d.c. motor using a magnetic field by a permanent magnet. Numericalreference 4 designates a bridge circuit for reversibly driving the motor3; numerical reference 5 designates a motor current detecting circuitfor detecting a current of the motor 3; numerical reference 6 designatesa relay for allowing or prohibiting a supply of an electric current tothe first converter 2 or the bridge circuit 4; and numerical reference 7designates an interface circuit for connecting the above-mentionedvarious circuits to a micro controller 8.

The motor 3 is appropriately controlled by the micro controller 8depending on steering conditions by a driver. Hereinbelow, an operationof the micro controller 8 will be described in reference of the flowchart in FIG. 2. A program in FIG. 2 is stored in an ROM (not shown),built in the micro controller 8, and periodically read out by an upperprogram.

In Step S1, a motor current is subjected to a feedback control based ona target electric current of the motor, set by a steering torque, avehicle speed, and so on so that the target electric current is inagreement with a detection electric current by the motor currentdetecting circuit 5 in accordance with, for example, an algorithm suchas a PT control. This feedback control is known in a conventionaltechnique, and a detailed description is omitted.

In Step S2, by comparing an input voltage Vi of the first converter 2with an output voltage Vo of the first converter 2, a failure of thefirst converter 2 is judged. For example, when Vo is lower than Vi, ashort-circuit of the first switching element 22 and a failure such thatthe switching element 23 is not turned on are judged.

When the first converter 2 is normal, Step S3 is processed to judgewhether or not the steering speed is high. A deviation of the feedbackcontrol of the motor current is used to judge the steering speed. When(target value-detection value)>judging value is established, thesteering speed is judged high.

The judgment whether or not the steering speed is high will be describedin detail. In FIG. 3, a relationship between a rotational speed and anelectric current in a case that a power source voltage is constant andthe motor current of the d.c. motor using a magnetic field by a magnetis subjected to a feedback control so as to be a constant value I0 isshown. Because a counter-electromotive force by the motor is inproportion to the rotational speed, when the rotational speed exceedsN1, an impressed voltage to the motor is saturated at a voltagesubstantially the same as the power source voltage, and the motorelectric current is reduced as the rotational speed increases.Therefore, when the above deviation between the target value and thedetection value is a predetermined judging value or less, the rotationalspeed of the motor is N1 or more, whereby the steering speed is judgedhigh.

The relationship shown in FIG. 3 is determined by a characteristic ofthe motor. A motor having a small torque constant puts out a higheridling rotational speed, and a motor having a large torque constant putsout a lower idling rotational speed N0. Further, in order to obtain anidentical output torque, the motor having the small torque constantrequires a large electric current, and the motor having the large torqueconstant requires a small electric current.

As described, when the steering speed is high, in other words, a driverquickly steers, the output torque of the motor is decreased by adecrement of the motor current, whereby a steering force by the driveris increased. In order to relax an influence of this phenomenon, a motorhaving a small torque constant and putting out a higher idlingrotational speed N0 may be used. However, in this case, a largerelectric current is required to obtain the identical motor outputtorque, whereby an efficiency is deteriorated.

Accordingly, the motor having the larger torque constant and putting outthe smaller idling rotational speed N0 is used, wherein when thesteering speed is low, it is necessary to obtain a larger output torqueby a smaller electric current, and when the steering speed is high, itis necessary to prevent a drop of the motor current caused by acounter-electromotive force by boosting the power source voltage.Although an efficiency is lowered by a loss in the converter used forboosting the power source voltage, the steering speed is high when thedriver quickly steers as described above. In an actual drivingcondition, a frequency of quick steering is low, and therefore afrequency of operating the converter is low, whereby an efficiency isimproved on average under a normal driving condition.

In order to achieve the above-described purposes, in Steps S4 throughS7, the first converter 2 is operated when the steering speed is judgedhigh in Step S3. At first, in Step S4, the relay 6 is turned on. Next,the first switching element 22 is driven by pulse in Step S5, and thesecond switching element 23 is turned off in Step S6. The switchingelement is an MOSFET, by which a parasitic diode appearing because ofits structure rectifies, the first converter 2 is operated as a boostingchopper, and the boosted voltage is stored in the capacitor 24. Theboosted voltage is used to drive the motor 3 through the bridge circuit4 to cancel the counter-electromotive force by the boosted voltage,whereby the deviation of the motor current approaches 0.

In the next, the bridge circuit 4 is turned on in Step S7. As describedbelow, when the steering speed is a predetermined value or less, themotor current is subjected to a feedback control by a so-called PWMdrive. However, when the steering speed is judged high in Step S3, themotor current does not reach the target value even though the bridgecircuit 4 is driven at a duty ratio of 100%. In this case, the bridgecircuit 4 is driven at the duty ratio of 100%. Accordingly, it ispossible to render a switching loss of the bridge circuit 4 zero, and anefficiency is further improved.

Succeedingly, Steps S8 through S11 are a process when the steering speedis judged low in Step S3. As described, the motor current is subjectedto the feedback control by the PWM drive of the bridge circuit 4.

At first, a relay 6 is turned on in Step S8. Next, in Step S9, the firstswitching element 22 is turned off, and the second switching element 23is turned on in Step S10, whereby a loss in the switching element 23 issuppressed. Finally, in Step S11, the bridge circuit 4 is driven by PWMat a predetermined duty ratio. By the above operations, even though thevoltage is not boosted, the feedback control of the motor current isachieved while suppressing a loss of the first converter 2.

Finally, Steps S12 through S15 are processed in a case that the firstconverter 2 is judged to be in failure in Step S2. At first, the relay 6is turned off in Step S12, whereby it is possible to prevent fire and Sso on from occurring in a case such that the switching element 22 isdisordered by a short circuit. Further, in Step S13, the first switchingelement 22 is turned off, and the second switching element 23 is turnedoff in Step S14, whereby the motor 3 is securely turned off. Finally, inStep S15, the bridge circuit 4 is turned off.

Next, a driving circuit of the bridge circuit 4 will be described. FIG.4 illustrates a structure of the driving circuit of the bridge circuit4. The same numerical references are used for the same portions of FIG.1, and description of these portions are omitted. Numerical references71 through 73 designate parts of an interface circuit 7, whereinnumerical reference 71 designates a switching element driving circuit ona power source side for driving MOSFET on the power source side, formingthe bridge circuit 4; numerical reference 72 designates a switchingelement driving circuit on a ground side for driving MOSFET on theground side; and numerical reference 73 designates a second converter,being a power source of the switching element driving circuit on thepower source side 71. Although only a driving circuit for a single armof the bridge circuit 4 is illustrated in FIG. 4, a driving circuit forthe other arm is located in a manner similar thereto.

As illustrated in FIG. 4, the bridge circuit 4 is formed by N-channelMOSFET. The MOSFET on the power source side has a source potential equalto an output voltage Vo of the converter 2, wherein it is necessary toapply a voltage higher than Vo to a gate. Accordingly, the outputvoltage of the first converter 2 is connected to the second converter 73to boost Vo and is supplied as a power source for the switching elementdriving circuit on the power source side 71. On the other hand, becausethe source of the MOSFET on the ground side has a ground potential, thebattery 1 is supplied as a power source for the switching elementdriving circuit on the ground side 72.

By constructing the driving circuit of the bridge circuit 4 as above, itis possible to turn on and off MOSFET forming the bridge circuit 4 withsufficient stability. Further, since the second converter is connectedto the MOSFET on the power source side, it is not required to locate anyunnecessary converter on the ground side, whereby a system with a highefficiency can be constructed.

As described above, the steering controlling device according to thisembodiment generates a sufficient motor output torque even though thesteering speed is high and demonstrates a high efficiency. Although thenumber of elements is reduced and a loss at the time of stopping theboost is reduced using the parasitic diode of the MOSFET as therectifier diode and turning on and off the MOSFET in response tostarting and stopping of the boost, the converter is not limited to thiscircuit structure, and other structures may demonstrate theabove-mentioned effects.

Further, although the boost is immediately started when the deviation ofthe motor current is the predetermined value or more in the aboveembodiment, the boost may be started by the first converter 2 when theabove condition continues for a predetermined time. In accordancetherewith, it is possible to prevent hunting at a time of judgingconditions for boosting.

Further, although the boost is started by the first is converter 2 when(target value-detection value)>judging value is established and theboost by the first converter 2 is stopped when (target value-detectionvalue)≦judging value is established, a structure that a hysteresis isgiven to a referee condition and a second judging value smaller than theabove judging value, i.e. (target value-detection value) or more, isadopted as a condition for stopping the boost by the first converter 2,may be used. In accordance therewith, it is possible to prevent huntingat the time of judging the boosting condition.

Further, although the PWM duty ratio of the bridge circuit 4 is 100% atthe time of starting the boost by the first converter 2, the PWM drivemay be continued in response to a result of an operation of the currentfeedback control S1. In accordance therewith, it is possible to controlthe motor current with high accuracy when the steering speed is high.

Further, the judging value for judging the steering speed in Step S3 maybe changed in response to the power source voltage, the motor current,and so on.

Further, as known from FIG. 3, N1 increases as I1 decreases, and themotor current is not lower than the target value under a further highersteering speed. In other words, as the load of the motor is high and themotor current is large, the motor current is lower than the target valueunder a further low steering speed, whereby a lack of the output torquefrom the motor is further conspicuous under a low-speed drive. Inaccordance therewith, a structure that the judgment whether or not theboost is necessary based on the judgment of the rotational speed may beadopted only under the low-speed drive. In this case, unnecessaryoperations of the converter during the drive can be suppressed, the lossis further reduced, and a highly efficient system can be constructed.

Further, although the motor is stopped to drive when the first converter2 is judged to be in failure in Step S2 in the above embodiment, astructure that only the boosting operation is stopped and the motor iscontinued to be driven may be adopted. Further, when the converter is infailure, it may be possible to alert the driver by a warning lamp or thelike.

Further, although any sensor for detecting a steering angle and therotational speed of the motor is not used, and it is judged whether ornot the rotational speed of the motor is high based on the deviationbetween the target value and the motor current so as to demonstrateeffects such as a compactness of the system and a cost reduction inaddition to the above-mentioned effects of improving the systemefficiency in the above embodiment, a structure that the above sensorsare located to judge whether or not the boost is necessary based on arotational speed obtained from these sensors may be adopted if only aneffect of improving a system efficiency, which is the most importance inthe present invention.

Embodiment 2

Although the first converter 2 is started and stopped based on only theresult of the operation of the motor current feedback control in theabove embodiment, the output voltage from the first converter may be setto be a predetermined value by a feedback control to obtain a constantvoltage. Accordingly, a voltage after boosting is stabilized,controllability of the motor is further improved, and steering feelingis improved in a power steering device.

Further, the first converter 2 may be driven so that a voltage Vo afterboosting is a predetermined value or less. Accordingly, it is possibleto protect various circuit, to which the output voltage Vo from thefirst converter 2 is applied, from a high voltage.

Embodiment 3

Although it is judged whether or not the boosting operation is necessarybased on the deviation of the electric current in the above embodiment,necessity of a boosting operation may be judged when an impressedvoltage to a motor is a predetermined value or more. In other words, theimpressed voltage is high in a case that a motor current feedbackcontrol indicates a PWM duty of about 100% by an influence of acounter-electromotive force of the motor. In this case, in order toobtain a desirable motor output torque, it is necessary to furtherincrease the impressed voltage. Therefore, when the impressed voltage isthe predetermined value or more, the boosting operation is conducted inthis case, a voltage is boosted before a current deviation is generated,whereby steering feeling can be further improved.

Further, by operating the counter-electromotive force from the impressedvoltage and a motor current and judging that a rotational speed is highwhen the counter-electromotive force is a predetermined value or more,the judgment is further accurate.

The first advantage of the steering controlling device according to thepresent invention is that because the steering controlling devicecomprises the power source, the motor connected to the steering system,the motor driving circuit converting the power source voltage to thepredetermined voltage and applying the voltage to the motor, and thecontrolling means for controlling the motor driving circuit, wherein thecontrolling means controls the motor driving circuit so as to boost thepower source voltage when the rotational number of the motor is thepredetermined value or more, it is possible to boost the power sourcevoltage only in the case that the torque, generated from the motor, isshort by an influence of an counter-electromotive force, generated by ahigh rotational number of the motor actually requiring a boost of thepower source voltage, and therefore the efficiency of the entire devicecan be improved by reducing the frequency of the boost.

The second advantage of the steering controlling device according to thepresent invention is that because the controlling means operates thenumber of revolutions of the motor based on the electric current of themotor and the impressed voltage, it is possible to judge whether or notthe rotational speed of the motor is high without specifically locatinga rotation sensor and so on, and effects such as compactness of thesystem and a reduction of the cost can be demonstrated.

The third advantage of the steering controlling device according to thepresent invention is that because the steering controlling devicecomprises the power source, the motor connected to the steering system,the motor driving circuit converting the power source voltage to thepredetermined voltage and applying the voltage to the motor, and thecontrolling means for controlling the motor driving circuit, wherein thecontrolling means controls the electric current of the motor so as to bethe predetermined target value and controls the motor driving circuit soas to drive the motor by boosting the power source voltage when themotor current is lower than the target value, it is possible to boostthe power source voltage only in the case that the torque generated bythe motor is short by an influence of a counter-electromotive forcecaused by a high rotational number of the motor actually requiring theboost of the power source voltage, an effect of improving the efficiencyof the entire device is obtainable by reducing a frequency of the boost,it is possible to judge whether or not the rotational number of themotor is high without specifically locating any rotational sensor and soon, and the effects such as a compactness of the system and a reductionof the cost can be demonstrated.

The fourth advantage of the steering controlling device according to thepresent invention is that because the steering controlling devicecomprises the power source, the motor connected to the steering system,the motor driving circuit converting the power source voltage to thepredetermined voltage and applying the voltage to the motor, and thecontrolling means for controlling the motor driving circuit, wherein thecontrolling means controls the electric current of the motor so as to bethe predetermined target value and controls the motor driving circuit soas to drive the motor by boosting the power source voltage when theimpressed voltage to the motor is the predetermined value or more, it ispossible to boost the power source voltage only in the case that thetorque generated by the motor is short by an influence of acounter-electromotive force caused by a high rotational number of themotor actually requiring the boost of the power source voltage, theeffect that efficiency of the entire device is improved by reducing thefrequency of the boost can be demonstrated, it is possible to judgewhether or not the rotational number of the motor is high withoutspecifically locating any rotational sensor and so on, the effects suchas a compactness of the system and a reduction of the cost can bedemonstrated, and further because the voltage is boosted before acurrent deviation is generated, it is possible to further improve thesteering feeling.

The fifth advantage of the steering controlling device according to thepresent invention is that because the motor driving circuit iscontrolled to drive the motor by boosting the power source voltage inthe case that the referee condition for boosting the power sourcevoltage continues for the predetermined time or more, it is possible todemonstrate the effect that hunting in judging whether or not thevoltage is boosted can be prevented.

The sixth advantage of the steering controlling device according to thepresent invention is that because the motor uses a magnetic field by thepermanent magnet, it is possible to obtain a large output torque by afurther small electric current in the case that the steering speed islow, and an effect of improving an efficiency of the entire device canbe demonstrated.

The seventh advantage of the steering controlling device according tothe present invention is that because the motor driving circuit isformed by at least the first converter connected to the power sourcevoltage and the bridge circuit connected to the motor, an effect thatthe steering controlling device effecting forward and backward rotationsof the motor is easily constructed can be demonstrated.

The eighth advantage of the steering controlling device according to thepresent invention is that because the second converter for furtherboosting the output voltage from the first converter in order to drivethe switching elements, forming the bridge circuit, is located, aneffect of turning on and off the switching element forming the bridgecircuit with sufficient stability can be demonstrated.

The ninth advantage of the steering controlling device according to thepresent invention is that only the switching element on the power sourceside among the switching elements forming the bridge circuit is drivenby the second converter, an effect that the system having a further highefficiency is constructed without locating any unnecessary converter onthe ground side can be demonstrated.

The tenth advantage of the steering controlling device according to thepresent invention is that because the controlling means monitors atleast the input voltage of the first converter, an effect that a failureof the first converter is detected and an appropriate treatment suchthat the first converter is cut off is conducted when the failure occurscan be demonstrated.

The eleventh advantage of the steering controlling device according tothe present invention is that because the controlling means monitors atleast the output voltage from the first converter, an effects that afailure of the first converter is detected and an appropriate treatmentsuch that the first converter is cut off is conducted when the failureoccurs can be demonstrated.

The twelfth advantage of the steering controlling device according tothe present invention is that because the controlling means controls theoutput voltage from the first converter so as to be the predeterminedtarget value, effects that the voltage after the boost is stabilized,controllability of the motor is improved, and the steering feeling ofthe power steering device is improved can be demonstrated.

The thirteenth advantage of the steering controlling device according tothe present invention is that because the controlling means controls theboosted voltage so as to be the predetermined value or less, it ispossible to prevent the first converter from generating an unnecessaryhigh voltage, and the bridge circuit and so on, to which the outputvoltage from the first converter is supplied, can be protected from thegenerated high voltage.

The fourteenth advantage of the steering controlling device according tothe present invention is that because the controlling means stops thefirst boosting operation when the first converter is judged abnormal, itis possible to prevent the elements forming the first converter fromburning, smoking, firing, and so on, accompanied by the burning, can bepreviously prevented.

The fifteenth advantage of the steering controlling device according tothe present invention is that the controlling means stops an applicationof the electric current to the motor when the first converter is judgedabnormal, the appropriate fail safe treatment of stopping the control ofthe system can be conducted when a failure is caused.

The sixteenth advantage of the steering controlling device according tothe present invention is that because the power source is connected tothe motor driving circuit through the switching means, and a contact ofthe switching means is opened when it is judged that the first converteris abnormal, it is possible to previously prevent firing and so on incases such that the switching element forming the motor driving circuitare short-circuited and disordered.

The seventeenth advantage of the steering controlling device accordingto the present invention is that because the duty ratio of driving theswitching elements of the bridge circuit is 100% in the case that thefirst converter is operated to boost the voltage, it is possible torender the switching loss of the bridge circuit 0, and the efficiencycan be further improved.

The eighteenth advantage of the steering controlling device according tothe present invention is that because terminals of the rectifier diodesforming the first converter are shorted in the case that the firstconverter is not operated to boost the voltage, it is possible toprevent a loss by the elements forming the first converter.

The nineteenth advantage of the steering controlling device according tothe present invention is that because the rectifier diodes forming thefirst converter are the parasitic diodes of MOSFET, the number of theelements can be reduced, and a loss caused at the time of stopping theboost can be reduced.

The twentieth advantage of the steering controlling device according tothe present invention is that because the controlling means controls themotor current so as to be the predetermined target value by driving atleast one of the first converter and the bridge circuit by pulse, anappropriate current control can be constantly achieved.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A steering controlling device comprising: a power source; a motorconnected to a steering system; a motor driving circuit converting apower source voltage to a predetermined voltage and applying theconverted voltage to the motor; a means for controlling the motordriving circuit, wherein the controlling means controls the motordriving circuit to boost the power source voltage when a rotationalspeed of the motor is greater than a predetermined value; wherein themotor driving circuit includes at least a first converter connected tothe power source and a bridge circuit connected to the motor; and asecond converter boosting an output voltage from the first converter inorder to drive switching elements, forming the bridge circuit.
 2. Thesteering controlling device according to claim 1, wherein thecontrolling means controls the motor driving circuit by controlling anelectric current of the motor to be a predetermined target value andboosting the power source voltage when a voltage applied to the motorexceeds a predetermined value to drive the motor.
 3. The steeringcontrolling device according to claim 1, wherein the motor drivingcircuit is controlled to boost the power source voltage in order todrive the motor when a condition for boosting the power source voltageis satisfied for a predetermined time or more.
 4. The steeringcontrolling device according to claim 1, wherein the motor uses amagnetic field by a permanent magnet.
 5. The steering Controlling deviceaccording to claim 1, wherein the switching elements on a side of thepower source are driven by the second converter.
 6. The steeringcontrolling device according to claim 1, wherein the controlling meansmonitors an input voltage to the first converter or an output voltagefrom the first converter.
 7. The steering controlling device accordingto claim 1, wherein the controlling means controls an output voltagefrom the first converter to be a predetermined target value.
 8. Thesteering controlling device according to claim 7, wherein thecontrolling means controls the boosted voltage to be a predeterminedvalue or less.
 9. The steering controlling device according to claim 1,wherein a duty ratio of driving the switching elements of the bridgecircuit is 100% when the first converter is operated to boost thevoltage.
 10. The steering controlling device according to claim 1,wherein the diodes forming the first converter are parasitic diodes ofMOSFET.
 11. The steering controlling device according to claim 1,wherein the controlling means controls the electric current of the motorto be a predetermined target value by driving at least one of the firstconverter and the bridge circuit by pulse.
 12. The steering deviceaccording to claim 1, wherein the controlling means operates therotational speed of the motor based an electric current and an impressedvoltage of the motor.